Many of the cobalt and nickel based superalloy materials traditionally used to fabricate the majority of combustion turbine components used in the hot gas path section of the combustion turbine engine are insulated from the hot gas flow by coating the components with a thermal barrier coating (TBC) in order to survive long term operation in this aggressive high temperature combustion environment.
TBC systems often consist of four layers: the metal substrate, a McrAlY (where M is Ni, Co or a combination thereof) bond coat, a thin thermally grown oxide (TGO) formed during service, and ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ), which is desirable for having very low thermal conductivity while remaining stable at nominal operating temperatures typically seen in applications. TBCs experience degradation through various degradation modes that include mechanical rumpling of bond coat during thermal cyclic exposure, accelerated oxidation of bond coat, hot corrosion, and molten deposit degradation. With the loss of the TBC, the component experiences much higher temperatures and the component life is reduced dramatically.
Typical MCrAlY bond coats have a 2-phase structure of fine γ-(M) (face-center cubic) and β-(M)Al (body-center cubic). The β-(M)Al phase is the aluminum (Al) reservoir. Aluminum in the bond coat will be depleted during service by either diffusion to the bond coat/TBC interface forming α-Al2O3 TGO or into the substrate. Spallation of the TBC will occur when the TGO layer is very thick or there is no more aluminum from the bond coat to form the adherent α-Al2O3 scale. Aluminum diffusion and TGO growth depend on bond coat temperatures, i.e., higher bond coat temperatures will accelerate aluminum diffusion and TGO growth, and hence TBC spallation and reduce component service life. Therefore, bond coat temperatures are limited due to oxidation, spallation, and depletion of the aluminum reservoir in the bond coating. Some standard bond coatings, such as GT33 (available from Sulzer Metco, Westbury, N.Y.) have operational temperature limits of about 1750° F. At temperature higher than the operational temperature limit, the bond coating will deteriorate much faster due to accelerated oxidation which increases the chances of spallation of the thermal barrier coating (TBC) applied to the bond coating and hence reduce the component service life.
Therefore, a bond coating powder and method of making the bond coating powder and method of applying the bond coating powder to components in gas turbines that do not suffer from the above drawbacks is desirable in the art.